Provided is a power assist device, which solves the conventional problem that, when load power is assisted, the number of components increases because an AC power detector is provided in the AC power supply of an inverter. An instantaneous output power calculating section consists of a power calculating section for calculating power based on the detection values of a current and a voltage on the output side of an inverter and a correction calculating section for correcting the calculated power. Additionally, limiter sections are provided in charging and discharging control units of a chopper, respectively, and the values of the limiters are varied with the power calculated by the instantaneous output power calculating section. The charging and discharging control units are configured such that a DC detected voltage of the inverter is inputted to obtain deviation signals between the DC detected voltage and preset upper and lower limit voltages and the deviation signals are inputted to the voltage control unit to calculate the value of a charging command.